Fabrication of three-dimensional metallic components via layer-by-layer laser cladding was first reported in 1978 by Breinan and Kear. In 1982, U.S. Pat. No. 4,323,756 issued to Brown et al., which describes a method for the production of bulk, rapidly solidified metallic articles, finding particular utility in the fabrication of certain gas turbine engine components including discs and knife-edge air seals.
Recently, various groups around the world have been working on different types of layered manufacturing techniques for fabrication of near-net-shape metallic components. Recent innovations include the integration of lasers with multi-axis CNC machines and co-axial nozzles toward the fabrication of three-dimensional components.
However, previous approaches are all open-loop processes requiring either a considerable amount of periodic machining or final machining to achieve close dimensional tolerances. Continuous corrective measures during the manufacturing process are necessary to fabricate net shape functional parts with close tolerances and acceptable residual stress.
U.S. Pat. No. 6,122,564, the entire contents of which are incorporated herein by reference, describes a laser-based, direct metal deposition fabrication process capable of producing near net-shape, fully dense molds, dies, and precision parts, as well as engineering changes or repairs to existing tooling or parts. According to the process, an industrial laser beam is focused onto a workpiece, creating a melt pool into which powdered metal is injected. The beam is moved under CNC control, based on a CAD geometry, tracing out the part, preferably on a layer-by-layer basis. Optical feedback is preferably used to maintain tight closed-loop control over the process.
Initial data using an optical feedback loop along with a CNC working under the instructions from a CAD/CAM software, indicate that closed-loop DMD can be used to produce three-dimensional components directly from the CAD data, thereby eliminating intermediate machining and considerably reducing the amount of final machining. This technology is now being commercialized, with surface finishes on the order of 100 micron being routinely achievable. In addition to close-dimensional tolerances, the closed-loop DMD process enables fabrication of components with multiple materials.
At present, the DMD system utilizes either a high-power CO2 or Nd-YAG laser. Closed-loop control using the optical feedback response partly depends on the laser pumping mechanism and its response to the signal from the feedback loop. For a CO2 laser, response time is influenced by the relaxation behavior of the gas medium and excitation mechanism such as radio frequency (RF) or direct current. Carbon dioxide and nitrogen gases in a CO2 laser exhibit minimal response to a signal above 25 kHz; even above 5 KHz, response is rather sluggish.
Thus, the need remains for a system and method to improve the response time in laser-assisted direct metal deposition processes.